Transmission shift with pressure controlled cleaning pulses

Abstract

A transmission includes a gearing arrangement configured to shift speed ratios by disengaging an off-going shift element and engaging an oncoming shift element and an electrohydraulic valve having an actuatable valve element configured to control an engagement state of the oncoming shift element. A controller is programmed to, during a shift of the transmission, in response to the off-going shift element disengaging and an expected ratio change not initiating, monotonically increase current to the valve to a maximum value to overcome frictional resistance on the valve element, and programmed to, in response to expiration of a timer and the ratio change still not initiating, send a repeating pattern of high and low current signals according to a duty cycle to overcome frictional resistance of the valve element.

What is claimed is: 
     
       1. A transmission comprising:
 an input shaft; 
 an output shaft; 
 a plurality of shift elements engageable in sets to create a plurality of power flow paths between the input and output shafts at different speed ratios; 
 a hydraulic system in fluid communication with the shift elements and including at least one electrohydraulic valve configured to control fluid pressure to at least one of the shift elements; and 
 a controller in communication with the at least one valve and programmed to, during a shift of the transmission:
 command a series of first pressures to a first of the valves associated with an oncoming one of the shift elements while an off-going one of the shift elements is engaged, 
 command a series of second, higher pressures to the first valve for a duration of time responsive to an expected ratio change between the input and output shafts not commencing and the off-going shift element slipping, and 
 command a series of pressure pulses, according to a duty cycle, to the first valve responsive to expiration of the duration of time and the expected ratio change not commencing. 
 
 
     
     
       2. The transmission of  claim 1 , wherein the duty cycle is predetermined. 
     
     
       3. The transmission of  claim 1 , wherein the pressure pulses include a high pulse and a low pulse, and wherein the high pulse is a maximum pressure associated with the first valve. 
     
     
       4. The transmission of  claim 3 , wherein the duty cycle is at least 50 percent high pulse. 
     
     
       5. The transmission of  claim 3 , wherein the low plus is equal to an initial pressure of the series of first pressures. 
     
     
       6. The transmission of  claim 1 , wherein at least one of the series of second, higher pressures is equal to a maximum pressure associated with the first valve. 
     
     
       7. The transmission of  claim 1 , wherein the series of second pressures increases monotonically. 
     
     
       8. The transmission of  claim 1 , wherein the series of first and second pressures are commanded for a predetermined duration of time. 
     
     
       9. The transmission of  claim 1 , wherein the controller is further programmed to, responsive to the series of pressure pulses resulting in a ratio change between the input and output shafts, command a series of third pressures to the first valve that is less than the series of second pressures to engage the oncoming clutch. 
     
     
       10. The transmission of  claim 1 , wherein the controller is further programmed to, responsive to the series of second pressures resulting in a ratio change between the input and output shafts, command a series of third pressures to the first valve that is less than the series of second pressures to engage the oncoming clutch. 
     
     
       11. The transmission of  claim 1 , wherein the controller is further programmed to, responsive to the series of pressure pulses not resulting in a ratio change between the input and output shafts within a duration of time, abandon the shift. 
     
     
       12. A method of freeing a stuck electrohydraulic valve of a transmission, the method comprising:
 commanding a series of first pressures to an electrohydraulic valve associated with an oncoming shift element of a transmission shift prior to an off-going shift element slipping; 
 commanding a series of second, higher pressures to the valve for a duration of time responsive to an expected ratio change of the transmission not commencing and the off-going shift element slipping; and 
 commanding a series of pressure pulses, according to a duty cycle, to the valve responsive to expiration of the duration of time and the expected ratio change not commencing. 
 
     
     
       13. The method of  claim 12  further comprising:
 responsive to the series of pressure pulses resulting in a ratio change of the transmission, commanding a series of third pressures to the valve that is less than the series of second pressures. 
 
     
     
       14. The method of  claim 12  further comprising:
 responsive to the series of second pressures resulting in a ratio change of the transmission, commanding a series of third pressures to the first valve to is less than the series of second pressures. 
 
     
     
       15. The method of  claim 12  further comprising:
 responsive to the series of pressure pulses not resulting in a ratio change of the transmission within a duration of time, aborting the shift. 
 
     
     
       16. The method of  claim 12  further comprising:
 responsive to the series of pressure pulses not resulting in a ratio change of the transmission within a duration of time, issuing a diagnostic code. 
 
     
     
       17. The method of  claim 12 , wherein the series of second pressures increases monotonically. 
     
     
       18. A transmission comprising:
 a gearing arrangement configured to shift speed ratios by disengaging an off-going shift element and engaging an oncoming shift element; 
 an electrohydraulic valve including an actuatable valve element configured to control an engagement state of the oncoming shift element; and 
 a controller programmed to, during a shift of the transmission:
 in response to the off-going shift element disengaging and an expected ratio change not initiating, monotonically increase current to the valve to a maximum value to overcome frictional resistance on the valve element, and 
 in response to expiration of a timer and the ratio change still not initiating, send a repeating pattern of high and low current signals according to a duty cycle to overcome frictional resistance of the valve element. 
 
 
     
     
       19. The transmission of  claim 18 , wherein high current signal is at a maximum value. 
     
     
       20. The transmission of  claim 18 , wherein the controller is further programmed to, responsive to the pattern of high and low current signals not producing the expected ratio change within a duration of time, abort the shift.